In a water-based bitumen extraction process, the efficiency of bitumen separation from mined oil sand is primarily determined by bitumen liberation from the sand grains to form bitumen droplets and bitumen recovery through coalescence, aeration and flotation of the bitumen droplets. Generally, aeration occurs due to natural entrainment of air and mechanical agitation of the water and oil sand slurry. The air bubbles present in an oil sand slurry are generally of a micro-size.
There is a strong correlation between bitumen droplet size and bitumen recovery. In general, good bitumen recovery could be obtained if the average size of bitumen droplets is greater than 400 μm. On the other hand, the recovery may be very poor if the average size of bitumen droplets is smaller than 200 μm. For small bitumen droplets (e.g., ≤200 μm), flotation is challenging.
When a bitumen droplet is very small, its probability of collision with another droplet for coalescence and with an air bubble for aeration is very low. This is because a small droplet does not possess sufficient kinetic energy to deviate from the streamlines and to displace the intervening liquid layer to collide with other bitumen droplets and/or bubbles. In addition, the interaction forces between bitumen droplets and between bitumen droplets and air bubbles are naturally repulsive due to the operation conditions normally used (slurry pH in the range of 7.5 to 9.5). The presence of strong repulsive forces makes coalescence and aeration difficult. Thus, small bitumen droplets do not readily float to the top of a separation vessel to be recovered as bitumen froth.
With respect to mined oil sands, it has been found that for oil sands that do not respond well to processing, referred to as problem ores or poor processing ores (e.g., some low-grade, high-fines ores), the average size of the bitumen droplets generated by the existing bitumen extraction processes is often small, leading to poor extraction performance. In order to improve the overall performance of a water-based bitumen extraction process especially for problem ores, the efficiency of bitumen droplets coalescence and aeration needs to be improved to increase the average size of final bitumen droplets.
In existing water-based bitumen extraction processes, caustic is often used as a process aid to improve the overall performance. Caustic helps to release natural surfactants and affects surface properties of bitumen, sand, and clays. The use of caustic reduces the attachment of fine solid particles on bitumen surface (so called slime coating), thus facilitating the coalescence and aeration of bitumen droplets. However, the use of caustic increases the slurry pH and thus increases the repulsions between bitumen droplets and between bitumen droplets and air bubbles. The use of a conditioning step in existing extraction processes with mechanical energy input also helps the coalescence and aeration of bitumen droplets, improving the overall performance. However, when processing problem ores, the use of caustic and the existing conditioning step cannot provide enough improvement to obtain desirable performance.